In a newly published study from Australia that included people with and without early signs of multiple sclerosis (MS), sun exposure and vitamin D levels each predicted disease risk.

It has long been recognized that MS is a disease of latitudes, with high rates reported in colder climates and lower rates seen in tropical ones.

Some previous studies have also found low vitamin D levels to be associated with higher risk for MS. Others have suggested that taking vitamin D supplements may protect against the disease.

But there have also been suggestions that vitamin D -- which is produced by the body when the skin is exposed to sunlight -- was only part of the story, study researcher Robyn Lucas, PhD, tells WebMD.

In a study published last spring, researchers reported that sun exposure protected against an MS-like disease in genetically susceptible mice and that the protection was not fully explained by the mice’s vitamin D levels.

“We really can’t say at this point if vitamin D is important or if it is just a reflection of sun exposure,” she says.

Sunlight exposes the body to both ultraviolet A (UVA) and ultraviolet B (UVB) rays. Vitamin D production increases mainly with exposure to UVB, but Lucas says some of the protection against MS could come from UVA rays.

Sun Damage and MS

Published in the February issue of Neurology, the Australian study included 216 people with early symptoms of MS who had not yet been diagnosed and 395 people with no MS symptoms matched for age, sex, and area of residence.

All the study participants were asked about their level of sun exposure during different periods of their lives, and the researchers also measured blood levels of vitamin D and skin damage due to sun exposure.

One measure of sun-related skin aging, which involved taking silicone impressions of the backs of the participants’ hands, primarily measured UVA exposure.

People with the most evidence of skin damage from sun exposure were about 60% less likely to develop MS symptoms than people with the least sun damage.

Those with the highest vitamin D levels were also less likely to have early signs of MS than people with the lowest levels, but this difference did not fully explain the sun-related association.

U.S. National Multiple Sclerosis Society Vice President Nicholas LaRocca, PhD, says a major strength of the study was that it included people who had not yet been diagnosed with MS.

The research was funded by the National MS Society in collaboration with the National Health and Medical Research Council of Australia and MS Research Australia.

“The findings suggest that the relationship between sun exposure and multiple sclerosis may be more complicated than we originally thought,” LaRocca tells WebMD.

Sun Safety

Lucas and LaRocca agree that the public health message about sun exposure and MS risk is also complicated.

“A few minutes of [unprotected] sun exposure a few times a day appears to be pretty safe for most people and is probably good for health,” Lucas says.

But she adds that skin type, sun intensity, and other factors all come in to play when determining what constitutes “safe” sun exposure.

“People have to find a balance and we are not really in the position to tell them what that balance is,” Lucas says. “Even a single sunburn can increase someone’s risk for skin cancer.”